Peripheral Component Interconnect Express (PCI-E) is an example of a communication protocol in which the clock of two clients is not common, and each client uses its own clock reference. In this case, the receive side must implement a clock recovery mechanism in order to sample the received data in the right phase. An important part of the clock recovery mechanism is a Phase Interpolator (PI), a block which is responsible to shift the phase up and down according to time recovery controls, and sample the received data in the right phase. The PI block is capable of generating multiple phases, also called taps.
PI testing in manufacturing is very important for several reasons. The PI block is usually an analog block that is sensitive. It is also not trivial to check the PI block during a functional test, since not all the taps of the PI block are usually used. Finally, the quality of this block is essential to chip robustness in user platform.
One of the ways to test a PI block in production is to inject a signal with parts per million (PPM) into the PI block and verify that the receive path works well. The PPM data will force the PI block to go over all its taps. The disadvantages of this method are several. The need to inject the PPM data into a receive path usually requires an expensive tester with special capabilities or a unique ad hoc external instrument which is attached to the tester. Only a “life test” of the PI block is provided. Integral non-linearity (INL) and Differential non-linearity (DNL) parameters can't be found.
There is a need for improved circuits and methods for PI testing.